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Dissecting Round Trip Time on the
Slow Path with a Single Packet
Pietro Marchetta, Alessio Botta, Ethan Katz-Bassett, Antonio PescapèPietro Marchetta, Alessio Botta, Ethan Katz-Bassett, Antonio Pescapè
University of Napoli Federico II, Napoli, ItalyUniversity of Southern California, Los Angeles, CA, USA
Round Trip Time (RTT): time to send a data packet and
receive its response
• Often used to infer the status of the network or of a network
path (e.g. through ping)
• For deriving application performance, for detecting anomalies, etc.
Introduction
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• For deriving application performance, for detecting anomalies, etc.
Problem: RTT comprises all the delays experienced along
the forward and reverse path!
Which part of the network is giving which contribution to
the RTT?
Example scenario 1• You are in a corporate network, reaching the Internet through one
or multiple providers
• You are experiencing bad performance
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• You want to understand if the provider is responsible
• To file a ticket
• To switch provider
• …
Example scenario 2
• You are at home and observe anomalous performance
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• You would like to know if the issue has to do with your home network (e.g. your son using bandwidth-hungry apps)
• We want to dissect the RTT in its components in order to isolate the contributions of the different parts of the network
• Teasing apart the contributing factors of RTT values is hard!
In general…
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hard!
How to evaluate the relative impact of each subpathon the total experienced RTT?
• Traceroute and Ping?
We may do a traceroute towards the destination and observe the
various RTT reported by the tool
Dissecting the Round Trip Time: traceroute
AS2907
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AS7527
AS4675
Average
It is not uncommon to observe RTT of intermediate
hops higher than the RTT of the destination
Dissecting the Round Trip Time: ping
We may do a traceroute towards the destination and then ping an
intermediate hop and the destination, and compare the RTT
Destination RTT > Intermediate RTT
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Same problem here: the RTT to intermediate hops
may appear higher than the RTT to the destination
Destination RTT < Intermediate RTT
• The RTT to intermediate hops may appear higher than
the RTT to the destination for several reasons
1. Different probes experiencing different network conditions
2. Intermediate hop not part of the reverse path
What happens?
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3. Intermediate hop very slow answering
4. Forward path toward the intermediate hop not part of the
forward path toward the destination
• destination-based routing
5. …
Goal: dissect the RTT into two chunks, at one
router
• Approach: use a single IP Timestamp probe
• Setting
Our proposal
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• Setting
• dissects at a router that
• appears on both forward and reverse path
• honors the IP Timestamp option
• Lets the sender specify up to four IP to request
timestamp from
• The incoming option is replicated by the destination
inside the ICMP Echo Reply
• It has been used by several works recently for multiple
IP timestamp option
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• It has been used by several works recently for multiple
objectives (alias resolution, infer router CPU load, etc.)
D
Using a single packet equipped with the Timestamp option
Our proposal: how it works
Key Idea: the intermediate hop is
requested to insert one Timestamp
along the forward and reverse path
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S
W
TS1
Forward Path Reverse Path
Local clock at S
Local clock at W
Local clock at D
Using a single packet equipped with the Timestamp option
D
Our proposal: how it works
Key Idea: the intermediate hop is
requested to insert one Timestamp
along the forward and reverse path
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S
W
TS1
TW1
Forward Path Reverse Path
Local clock at S
Local clock at W
Local clock at D
D
Our proposal: how it worksUsing a single packet equipped with the Timestamp option
Key Idea: the intermediate hop is
requested to insert one Timestamp
along the forward and reverse path
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S
W
TS1
TW1
TD1
Forward Path Reverse Path
Local clock at S
Local clock at W
Local clock at D
D
Our proposal: how it worksUsing a single packet equipped with the Timestamp option
Key Idea: the intermediate hop is
requested to insert one Timestamp
along the forward and reverse path
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S
W
TS1
TW1
TD1
Forward Path Reverse Path
TD2
Local clock at S
Local clock at W
Local clock at D
D
Our proposal: how it worksUsing a single packet equipped with the Timestamp option
Key Idea: the intermediate hop is
requested to insert one Timestamp
along the forward and reverse path
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S
W
TS1
TW1
TD1
Forward Path Reverse Path
TD2
TW2Local clock at S
Local clock at W
Local clock at D
D
Our proposal: how it worksUsing a single packet equipped with the Timestamp option
Key Idea: the intermediate hop is
requested to insert one Timestamp
along the forward and reverse path
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S
W
TS1
TW1
TD1
TS2
Forward Path Reverse Path
TD2
TW2Local clock at S
Local clock at W
Local clock at D
D
Our proposal: how it worksUsing a single packet equipped with the Timestamp option
Key Idea: the intermediate hop is
requested to insert one Timestamp
along the forward and reverse path
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S
W
TS1
TW1
TD1
TS2
Forward Path Reverse Path
RTT (S,D)
TD2
TW2Local clock at S
Local clock at W
Local clock at D
D
Our proposal: how it worksUsing a single packet equipped with the Timestamp option
Key Idea: the intermediate hop is
requested to insert one Timestamp
along the forward and reverse path
RTT(W,D)
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S
W
TS1
TW1
TD1
TS2
Forward Path Reverse Path
RTT (S,D)
TD2
TW2Local clock at S
Local clock at W
Local clock at D
D
Our proposal: how it works
Key Idea: the intermediate hop is
requested to insert one Timestamp
along the forward and reverse path
Using a single packet equipped with the Timestamp option
RTT(W,D)
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S
W
TS1
TW1
TD1
TS2
Forward Path Reverse Path
RTT(S,W)
RTT (S,D)
TD2
TW2Local clock at S
Local clock at W
Local clock at D
• Simple approach
• Single packet
• Collects 6 timestamps, 2 from source, 2 from intermediate
hop and 2 from destination
• Standard ping tool
Our proposal: summary
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• Standard ping tool
• Works on the slow path
• It needs a compliant router along path
• Honors the timestamp option
• Appears on both forward and reverse path
Evaluation
Applicability of the proposed approach
• We evaluated how many nodes per path are available for dissecting the RTT (i.e. are compliant with our approach)
• And where they are located along the path
Two use cases as proof of concept
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Two use cases as proof of concept
• Understanding if an anomalous behavior is caused by the ISP (isolating the contribution of an entire AS)
• Understanding if an anomalous behavior is caused by the home network (isolating the contribution of this LAN)
Applicability: experiments performed
• We identified a set of compatible destinations
• Honor the timestamp option and are not extra-stampers
• 36% of 1.7M IP addresses probed are compliant
• We randomly selected one representative IP for each AS
• We have 3,133 distinct Ases
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• We have 3,133 distinct Ases
• We performed traceroute towards these destinations and
probed all intermediate hops from 116 PlanetLab nodes
• Our next results are based on a dataset that comprises
223, 548 distinct paths
Applicability: results 1/2
• About 77.4% of the paths
contain at least one
compliant node
• And 27.3% contain more
than four compliant nodes
• On average, we observed
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• On average, we observed
2.5 compliant nodes per
path
• On average, about 17% of
the nodes in each scanned
path are compliant
Applicability: results 2/2• We evaluated the hop distance
of the compliant nodes from the edges
• We evaluated number of compliant nodes on
the path p appearing within v
hops from the source or the
destination
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overall number of
compliant nodes
• About 72% of all the compliant nodes are within 5 hops from the
source or the destination, and about 15% within one hop
• Symmetry does not only happen toward the edges
Use casesUse cases
Assessing the contribution of an AS to the RTT
• We want to evaluate the contribution of our AS to the
RTT toward some destination
• The experiment has been run from PlanetLab
• We show the results of an AS from Japan
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AS2907
AS7527
AS4675
Results on AS contribution to the RTT
Destination RTT > Intermediate RTT
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• Difference between RTT to destination and to intermediate hop is always > 0
• The average contribution of AS2907 is 76.8%• It is 106% according to ping!
Destination RTT < Intermediate RTT
Assessing the contribution of the home network
• Inside a home network
• We found compatible home routers
• NETGEAR DGN2200v3
• We also found compatible second hops
• Allowed to isolate the contribution of the last mile
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• We evaluated the contribution of the home network and of the last mile
• We performed experiments during several days and we show the interesting results
Results on home network contribution 1/2
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• During an overloaded period, the RTT grew in median by 356% (from 69.8 ms to 249 ms)
• The home network always played a marginal role (4.7% of RTT in unloaded and 2.6% in overloaded)
• Monitoring the RTT of the last mile and of the home network
• We artificially induced congestion toward some destinations
Results on home network contribution 2/2
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destinations
• Congestion did not affect the RTT of the home network but that of the last mile
Conclusion
• We presented an approach to dissect the RTT on the slow path
• Other techniques based on ping and traceroute may provide misleading results
• Our approach uses a single packet with the IP Timestamp option and requires a compliant router
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Timestamp option and requires a compliant router along the path
• A large-scale measurement study from 116 vantage points comprising 223K paths showed that 2.5 router per path on average are compliant
• We reported two use cases to show the possible uses of this approach
Thanks!!
Any question?Any question?
a.botta@unina.it
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